Ryo Ohshima

Strong evidence for d-electron spin transport at room temperature at a LaAlO3/SrTiO3 interface

A d-orbital electron has an anisotropic electron orbital and is a source of magnetism. The realization of a two-dimensional electron gas (2DEG) embedded at a LaAlO3/SrTiO3 interface surprised researchers in materials and physical sciences because the 2DEG consists of 3d-electrons of Ti with extraordinarily large carrier mobility, even in the insulating oxide heterostructure. To date, a wide variety of physical phenomena, such as ferromagnetism and the quantum Hall effect, have been discovered in this 2DEG system, demonstrating the ability of d-electron 2DEG systems to provide a material platform for the study of interesting physics. However, because of both ferromagnetism and the Rashba field, long-range spin transport and the exploitation of spintronics functions have been believed difficult to implement in d-electron 2DEG systems. Here, we report the experimental demonstration of room-temperature spin transport in a d-electron-based 2DEG at a LaAlO3/SrTiO3 interface, where the spin relaxation length is about 300 nm. Our finding, which counters the conventional understandings of d-electron 2DEGs, highlights the spin-functionality of conductive oxide systems and opens the field of d-electron spintronics.

Transport and spin conversion of multicarriers in semimetal bismuth

In this paper, we report on the investigation of (1) the transport properties of multi-carriers in semi-metal Bi and (2) the spin conversion physics in this semimetal system in a ferrimagnetic insulator, yttrium-iron-garnet. Hall measurements reveal that electrons and holes co-exist in the Bi, with electrons being the dominant carrier. The results of a spin conversion experiment corroborate the results of the Hall measurement; in addition, the inverse spin Hall effect governs the spin conversion in the semimetal/insulator system. This study provides further insights into spin conversion physics in semimetal systems.

Quantitative investigation of the inverse Rashba-Edelstein effect in Bi/Ag and Ag/Bi on YIG

The inverse Rashba-Edelstein effect (IREE) is a spin conversion mechanism that recently attracts attention in spintronics and condensed matter physics. In this letter, we report an investigation of the IREE in Bi/Ag by using ferrimagnetic insulator yttrium iron garnet. We prepared two types of samples with opposite directions of the Rashba field by changing a stacking order of Bi and Ag. An electric current generated by the IREE was observed from both stacks, and an efficiency of spin conversion—characterized by the IREE length—was estimated by taking into account a number of contributions left out in previous studies. This study provides a further insight into the IREE spin conversion mechanism: important step towards achieving efficient spin-charge conversion devices.

Significant reduction in spin pumping efficiency in a platinum/yttrium iron garnet bilayer at low temperature

The temperature evolution of a direct-current electromotive force (EMF) generated by spin pumping and the inverse-spin Hall effect in a platinum (Pt)/yttrium iron garnet (YIG) bilayer was investigated down to 80 K. The magnitude of the EMF decreased significantly with decreasing temperature and disappeared at approximately 80 K. 40-nm-thick YIG films fabricated by a metal organic decomposition method exhibited single-peak ferrimagnetic resonance (FMR) spectra without any spin wave resonance, which allowed us to precisely analyze the FMR spectra. We determined that the temperature evolution of the Gilbert damping constant is the dominant factor in the temperature dependence of the EMF. The comparison of the FMR linewidth between the X- and Q-bands revealed that an increase in Gilbert damping constant at low temperatures is not due to the enhancement of the spin pumping efficiency but due to an additional spin relaxation in the YIG film itself, which reduces the precession angle of the magnetization under the FMR conditions.

Note: Derivative divide, a method for the analysis of broadband ferromagnetic resonance in the frequency domain

Broadband ferromagnetic resonance (bbFMR) spectroscopy is an established experimental tool to quantify magnetic properties. Due to frequency-dependent transmission of the microwave setup, bbFMR measurements in the frequency domain require a suitable background removal method. Here, we present a measurement and data analysis protocol that allows us to perform quantitative frequency-swept bbFMR measurements without the need for a calibration of the microwave setup. We furthermore compare the results of the proposed frequency space analysis and a conventional analysis in field-space of bbFMR data obtained from a permalloy thin film. The very good agreement of the extracted parameters using the two methods shows the reliability of our method.

Spin-orbit coupling induced by bismuth doping in silicon thin films

We demonstrate an enhancement of the spin-orbit coupling in silicon (Si) thin films by doping with bismuth (Bi), a heavy metal, using ion implantation. Quantum corrections to conductance at low temperature in phosphorous-doped Si before and after Bi implantation is measured to probe the increase of the spin-orbit coupling, and a clear modification of magnetoconductance signals is observed: Bi doping changes magnetoconductance from weak localization to the crossover between weak localization and weak antilocalization. The elastic diffusion length, phase coherence length and spin-orbit coupling length in Si with and without Bi implantation are estimated, and the spin-orbit coupling length after the Bi doping becomes the same order of magnitude (Lso = 54 nm) with the phase coherence length (L{\phi} = 35 nm) at 2 K. This is an experimental proof that the spin-orbit coupling strength in Si thin film is tunable by doping with heavy metals.This study demonstrates an enhancement of spin-orbit coupling in silicon (Si) thin films by doping with bismuth (Bi), a heavy metal, using ion implantation. Quantum corrections to conductance at low temperatures in phosphorous-doped Si before and after Bi implantation are measured to probe the increase in spin-orbit coupling, and a clear modification of magnetoconductance signals is observed: Bi doping changes magnetoconductance from weak localization to the crossover between weak localization and weak antilocalization. The elastic diffusion length, phase coherence length, and spin-orbit coupling length in Si with and without Bi implantation are estimated, and the spin-orbit coupling length after Bi doping becomes the same order of magnitude (Lso = 54 nm) with the phase coherence length (Lφ = 35 nm) at 2 K. This is an experimental proof that spin-orbit coupling strength in the thin Si film is tunable by doping with heavy metals.This study demonstrates an enhancement of spin-orbit coupling in silicon (Si) thin films by doping with bismuth (Bi), a heavy metal, using ion implantation. Quantum corrections to conductance at low temperatures in phosphorous-doped Si before and after Bi implantation are measured to probe the increase in spin-orbit coupling, and a clear modification of magnetoconductance signals is observed: Bi doping changes magnetoconductance from weak localization to the crossover between weak localization and weak antilocalization. The elastic diffusion length, phase coherence length, and spin-orbit coupling length in Si with and without Bi implantation are estimated, and the spin-orbit coupling length after Bi doping becomes the same order of magnitude (Lso = 54 nm) with the phase coherence length (Lφ = 35 nm) at 2 K. This is an experimental proof that spin-orbit coupling strength in the thin Si film is tunable by doping with heavy metals.We demonstrate an enhancement of the spin-orbit coupling in silicon (Si) thin films by doping with bismuth (Bi), a heavy metal, using ion implantation. Quantum corrections to conductance at low temperature in phosphorous-doped Si before and after Bi implantation is measured to probe the increase of the spin-orbit coupling, and a clear modification of magnetoconductance signals is observed: Bi doping changes magnetoconductance from weak localization to the crossover between weak localization and weak antilocalization. The elastic diffusion length, phase coherence length and spin-orbit coupling length in Si with and without Bi implantation are estimated, and the spin-orbit coupling length after the Bi doping becomes the same order of magnitude (Lso = 54 nm) with the phase coherence length (L{\phi} = 35 nm) at 2 K. This is an experimental proof that the spin-orbit coupling strength in Si thin film is tunable by doping with heavy metals.

Spin injection into silicon detected by broadband ferromagnetic resonance spectroscopy

We studied the spin injection in a NiFe(Py)/Si system using broadband ferromagnetic resonance spectroscopy. The Gilbert damping parameter of the Py layer on top of the Si channel was determined as a function of the Si doping concentration and Py layer thickness. For a fixed Py thickness, we observed an increase in the Gilbert damping parameter with decreasing resistivity of the Si channel. For a fixed Si doping concentration, we measured an increasing Gilbert damping parameter for decreasing Py layer thickness. No increase in the Gilbert damping parameter was found for Py/Si samples with an insulating interlayer. We attribute our observations to an enhanced spin injection into the low-resistivity Si by spin pumping.